International Group of Researchers Measure Universe to 1 Percent Accuracy

PITTSBURGH—An international group of researchers, including physicists from Carnegie Mellon University’s McWilliams Center for Cosmology, have made the most precise calibration of the standard ruler that is used to measure the universe. The researchers, who are part of the Sloan Digital Sky Survey III’s (SDSS III) Baryon Oscillation Spectroscopic Study (BOSS), announced today at the 223rd Meeting of the American Astronomical Society (AAS) that they have used this standard ruler to measure the scale of the universe to an accuracy of 1 percent using galaxies more than six billion light years away.

“Before, our picture of the universe looked fuzzy. It was like we were nearsighted, but didn’t have glasses,” said Shirley Ho, assistant professor of physics at Carnegie Mellon. “Now we’re seeing 20/20 and we’re able to measure the scale of the universe’s structure to an accuracy of 1 percent. This can help us to better understand the expansion history of the universe and tell us vital information about the nature of the dark energy that drives the expansion.”

The distance and distribution of galaxies can be measured using what cosmologists call a standard ruler. Standard rulers can be thought of like a car’s headlights. The distance between the two headlights of most cars is more or less the same. The farther away a car is from the observer, the closer together the headlights appear to be. If we measure the angular separation between the headlights, we’re able to calculate how far away the car is. If we have multiple measurements, we can tell how fast the car is moving.

BOSS uses similar methods to measure baryon acoustic oscillations (BAO), relics of sound waves from the early universe that present themselves as ripples visible in the distribution of galaxies, as a standard ruler. Using the Sloan Foundation Telescope at the Apache Point Observatory in New Mexico, the BOSS researchers mapped more than 1.2 million light-emitting galaxies, and then used fundamental physics calculations to measure BAO. They then combined this data with measurements of temperature variation within the cosmic microwave background (CMB) radiation to reveal information about the acceleration of the expansion of the universe, and as a result, dark energy.

Ho, and Mariana Vargas-Magaña, a post-doctoral researcher at Carnegie Mellon’s McWilliams Center, led the portion of the project that focused on measuring the BAO standard ruler in two directions, parallel and perpendicular to the line of sight from the telescope (http://arxiv.org/abs/1312.4877). Ho also collaborated with researchers from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory to pair the two-dimensional sky coordinates with their redshifts to create a three-dimensional map of the density of galaxies in space, which allowed researchers to visualize the spacing of the galaxies and calculate the BAO.

“The observed spacing of galaxies is not the same in all directions, so we can’t assume that measuring a standard ruler in one direction would be sufficient. By measuring it in two directions we can create more stringent constraints,” Ho said. “And combining this measurement with what we know about the cosmic microwave background radiation, we can get the best understanding of our universe, and the dark energy that accelerates its expansion.”

Vargas-Magaña also led an in-depth analysis of the subtle effects that could distort or alter the BOSS measurements (http://arxiv.org/abs/1312.4996). This troubleshooting allowed the BOSS team to be confident in their measurements to the unprecedented 1 percent.

“When you’re dealing with something as large as the universe, there are many things that could interfere with you measurements,” Vargas-Magaña said. “To get to 1 percent, we needed to be meticulous about our measurements.”

Pictured above: An artist's concept of the new measurement of the size of the Universe. The gray spheres show the pattern of the "baryon acoustic oscillations" from the early Universe. Galaxies today have a slight tendency to align on the spheres -- the alignment is greatly exaggerated in this illustration. By comparing the size of the spheres (white line) to the predicted value, astronomers can determine to 1 percent accuracy how far away the galaxies are. (Image Credit: Zosia Rostomian, Lawrence Berkeley National Laboratory)

About Carnegie Mellon University: Carnegie Mellon (www.cmu.edu) is a private, internationally ranked research university with programs in areas ranging from science, technology and business, to public policy, the humanities and the arts. More than 12,000 students in the university’s seven schools and colleges benefit from a small student-to-faculty ratio and an education characterized by its focus on creating and implementing solutions for real problems, interdisciplinary collaboration and innovation. A global university, Carnegie Mellon has campuses in Pittsburgh, Pa., California’s Silicon Valley and Qatar, and programs in Africa, Asia, Australia, Europe and Mexico.

About the Sloan Digital Sky Survey: Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation and the U.S. Department of Energy Office of Science. The SDSS-III website is http://www.sdss3.org. SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington and Yale University.